Methods and catalysts for the manufacture of carbon fibrils

ABSTRACT

An improved catalyst for producing carbon fibrils is made by incorporating an effective yield-enhancing amount of a carboxylate into a fibril-forming catalyst. Alternatively, such a catalyst is made by coprecipitating a compound of a metal having fibril-forming catalytic properties and an aluminum and/or magnesium compound, optionally in the presence of carbon particles or carbon fibril aggregates. The catalyst may also be made by incorporating a compound of a fibril-forming metal onto magnesia particles in carbon particles or carbon fibril aggregates. The catalysts, methods of using them to form carbon fibrils and those carbon fibrils are also disclosed.

This is a continuation of U.S. Ser. No. 10/776,140, filed Feb. 11, 2004now U.S. Pat. No. 7,198,772, which is a continuation of U.S. Ser. No.09/783,173, filed Feb. 14, 2001, now U.S. Pat. No. 6,770,589, which is acontinuation of U.S. Ser. No. 08/464,278, filed Jun. 5, 1995, now U.S.Pat. No. 6,294,144, which is a division of U.S. Ser. No. 08/284,742,filed Aug. 2, 1994, now abandoned, which is a continuation of U.S. Ser.No. 07/887,307, filed May 22, 1992, now abandoned, all of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

Carbon fibrils are vermicular carbon deposits having diameters less than500 nanometers. They exist in a variety of forms, and have been preparedthrough the catalytic decomposition of various carbon-containing gasesat metal surfaces.

Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are freeof a continuous thermal carbon overcoat and have multiple graphiticouter layers that are substantially parallel to the fibril axis. Theygenerally have diameters no greater than 0.1 micron and length todiameter ratios of at least 5. Desirably they are substantially free ofa continuous thermal carbon overcoat, i.e., pyrolytically depositedcarbon resulting from thermal cracking of the gas feed used to preparethem.

Tubular fibrils having graphitic layers that are substantially parallelto the microfiber axis and diameters between 3.5 and 75 nanometers, arealso described in Tenant et al., U.S. Ser. No. 871,676 filed Jun. 6,1986 (“Novel Carbon Fibrils, Method for Producing Same and CompositionsContaining Same”), Tenant et al., U.S. Ser. No. 871,675 filed Jun. 6,1986 (“Novel Carbon Fibrils, Method for Producing Same and EncapsulatedCatalyst”), Snyder et al., U.S. Ser. No. 149,573 filed Jan. 28, 1988(“Carbon Fibrils”), Mandeville et al., U.S. Ser. No. 285,817 filed Dec.16, 1988 (“Fibrils”), and McCarthy et al., U.S. Ser. No. 351,967 filedMay 15, 1989 (“Surface Treatment of Carbon Microfibers”), all of whichare assigned to the same assignee as the present application and arehereby incorporated by reference.

Fibrils are useful in a variety of applications. For example, they canbe used as reinforcements in fiber-reinforced composite structures orhybrid composite structures (i.e. composites containing reinforcementssuch as continuous fibers in addition to fibrils). The composites mayfurther contain fillers such as a carbon black and silica, alone or incombination with each other. Examples of reinforceable matrix materialsinclude inorganic and organic polymers, ceramics (e.g., lead or copper).When the matrix is an organic polymer, it may be a thermoset resin suchas epoxy, bismaleimide, polyamide, or polyester resin; a thermoplasticresin; or a reaction injection molded resin. The fibrils can also beused to reinforce continuous fibers. Examples of continuous fibers thatcan be reinforced or included in hybrid composites are aramid, carbon,and glass fibers, alone, or in combination with each other. Thecontinuous fibers can be woven, knit, crimped, or straight.

The composites can exist in many forms, including foams and films, andfind application, e.g., as radiation absorbing materials (e.g., radar orvisible radiation), adhesives, or as friction materials for clutches orbrakes. Particularly preferred are fibril-reinforced composites in whichthe matrix is an elastomer, e.g., styrene-butadiene rubber,cis-1,4-polybutadiene, or natural rubber.

In addition to reinforcements, fibrils may be combined with a matrix tocreate composites having enhanced thermal, and/or electricalconductivity, and/or optical properties. They can be used to increasethe surface area of a double layer capacitor plate or electrode. Theycan also be formed into a mat (e.g., a paper or bonded non woven fabric)and used as a filter, insulation (e.g., for absorbing heat or sound),reinforcement, or adhered to the surface of carbon black to form “fuzzy”carbon black. Moreover, the fibrils can be used as an adsorbent, e.g.,for chromatographic separations.

Fibrils are advantageously prepared by contacting a carbon-containinggas with a metal catalyst in a reactor at temperature and otherconditions sufficient to produce them with the above-describedmorphology. Reaction temperatures are 400-850° C., more preferably600-750° C. Fibrils are preferably prepared continuously by bringing thereactor to the reaction temperature, adding metal catalyst particles,and then continuously contacting the catalyst with the carbon-containinggas.

Examples of suitable feed gases include aliphatic hydrocarbons, e.g.,ethylene, propylene, propane, and methane; carbon monoxide; aromatichydrocarbons, e.g., benzene, naphthalene, and toluene; and oxygenatedhydrocarbons.

Preferred catalysts contain iron and, preferably, at least one elementchosen from Group V (e.g., molybdenum, tungsten, or chromium), VII(e.g., manganese), or the lanthanides (e.g., cerium). The catalyst,which is preferably in the form of metal particles, may be deposited ona support, e.g., alumina and magnesia.

The carbon fibrils have a length-to-diameter ratio of at least 5, andmore preferably at least 100. Even more preferred are fibrils whoselength-to-diameter ratio is at least 1000. The wall thickness of thefibrils is about 0.1 to 0.4 times the fibril external diameter.

The external diameter of the fibrils preferably is between 3.5 and 75nanometers, i.e. determined by the particular application envisioned)have diameters within the range of 3.5-75 nanometers. Preferably a largeproportion have diameters falling within this range. In applicationswhere high strength fibrils are needed (e.g., where the fibrils are usedas reinforcements), the external fibril diameter is preferably constantover its length.

Fibrils may be prepared as aggregates having various macroscopicmorphologies (as determined by scanning electron microscopy) in whichthey are randomly entangled with each other to form entangled balls offibrils resembling bird nest (“BN”); or as aggregates consisting ofbundles of straight to slightly bent or kinked carbon fibrils havingsubstantially the same relative orientation, and having the appearanceof combed yarn (“CY”) e.g., the longitudinal axis of each fibril(despite individual bends or kinks) extends in the same direction asthat of the surrounding fibrils in the bundles; or, as, aggregatesconsisting of straight to slightly bent or kinked fibrils which areloosely entangled with each other to form an “open net” (“ON”)structure. In open net structures the degree of fibril entanglement isgreater than observed in the combed yarn aggregates (in which theindividual fibrils have substantially the same relative orientation) butless than that of bird nest. CY and ON aggregates are more readilydispersed than BN making them useful in composite fabrication whereuniform properties throughout the structure are desired. The substantiallinearity of the individual fibril strands also makes the aggregatesuseful in EMI shielding and electrical applications.

The macroscopic morphology of the aggregate is controlled by the choiceof catalyst support. Spherical supports grow fibrils in all directionsleading to the formation of bird nest aggregates Combed yarn and opennest aggregates are prepared using supports having one or more readilycleavable planar surfaces, e.g., an iron or iron-containing metalcatalyst particle deposited on a support material having one or morereadily cleavable surfaces and a surface area of at least 1 squaremeters per gram.

Preferred support materials include activated alumina or magnesia in theform of aggregates of tabular, prismatic, or platelet crystals. Suchmaterial is commercially available, e.g., from ALCOA (in the case ofactivated alumina) and Martin Marietta (in the case of magnesia). Theactivated alumina supports yield primarily combed yarn aggregates, whilethe magnesia supports yield primarily open net aggregates. Sphericalgamma alumina particles, which yield bird nest aggregates, are availablefrom Degussa.

It is believed that deposition of a catalyst on a support consisting ofreadily cleavable planar surfaces allows the fibrils to assist eachother as they grow, creating a “neighbor” effect. As the catalystparticles deposited on the flat surfaces initiate fibril growth, theindividual fibrils are influenced by their “neighbors”. In the case ofthe activated alumina support, this leads to a combed yarn fibrilaggregate in which the individual fibrils have the same relativeorientation. The magnesia supports, although having readily cleavableplanar surfaces, yield primarily lightly entangled, open net fibrilaggregates because they break apart more readily into smaller particlesthan the activated alumina support during fibril growth, resulting inaggregates that are less ordered than the combed yarn aggregates butmore ordered than the tightly entangled fibril balls. The oxideprecursors used to generate the metal catalyst particles also affect thetendency of the support to break apart. The more readily the oxide andsupport can form a mixed oxide at the interface between them, the morelikely the support is to break apart.

Further details regarding the formation of carbon fibril aggregates maybe found in the disclosure of Snyder et al., U.S. patent applicationSer. No. 149,573, filed Jan. 28, 1988, and PCT Application No. US89/00322, filed Jan. 28, 1989 (“Carbon Fibrils”) WO 89/07163, and Moy etal., U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989 andPCT Application No. US90/05498, filed Sep. 27, 1990 (“Fibril Aggregatesand Method of Making Same”) WO 91/05089, all of which are assigned tothe same assignee as the invention here and are hereby incorporated byreference.

Fibrils are increasingly important in a variety of industrial uses andwill become more so as these unique properties become better understoodand exploited. While known methods of manufacture permit production ofsmall quantities of fibrils, it is important to improve these methods,and in particular the catalysts used in those methods, to increase theyield of fibrils, to improve their quality and to lower their cost ofproduction. It is also desirable to produce carbon fibrils of improvedpurity.

OBJECTS OF THE INVENTION

It is thus a primary object of the invention to provide improvedcatalysts for the production of fibrils.

It is a further object of the invention to increase the yield andproductivity of fibril-producing catalysts.

It is still a further object of the invention to provide improvedmethods of preparing fibril-producing catalysts.

It is yet another object of this invention to improve the quality anduniformity of fibrils and aggregates thereof.

It is a further and related object of the invention to provide catalystswhich lend themselves to large-scale fibril-producing processes.

It is still a further and related object of the invention to improve theeconomics and reliability of fibril manufacture

SUMMARY OF THE INVENTION

Methods have now been found which substantially improve the yield ofcatalysts for the manufacture of carbon fibrils and carbon fibrilaggregates. Substantially improved yields can be obtained by contactinga fibril-forming catalyst with an effective yield-enhancing amount of acarboxylate, such as a lower carboxylic acid or a salt thereof. Themethod is preferably carried out by precipitating an effective amount ofa fibril-producing metal ion from an aqueous solution onto slurriedparticles of a support material in the presence of a carboxylate, suchas an anion of a water-soluble carboxylic acid, preferably having 1 to 4carbon atoms.

Another method which has been found to substantially improve themanufacture of catalysts for the production of carbon fibrils and fibrilaggregates includes the step of coprecipitating a compound of a metalhaving fibril-forming catalytic properties and an aluminum or magnesiumcompound under controlled pH conditions to form a catalyst comprising acompound of aluminum or magnesium and a compound of the metal(s).Desirably an aqueous solution of an iron and/or molybdenum salt and analuminum salt is formed and the metals coprecipitated to form a mixedoxide catalyst.

It has also been found that fibril-forming catalysts may beadvantageously supported on carbon particles and desirably on carbonfibril aggregates composed of carbon fibrils of particularcharacteristics. In these aggregates, a preponderance of the fibrilshave a length to diameter ratio of at least 5, an external diameter from3.5 to 75 nanometers and a wall thickness of 0.1 to 0.4 times theexternal diameter. The fibrils have graphitic layers substantiallyparallel to the fibril axis and are substantially free of pyrolyticallydeposited carbon. The active fibril-forming metal is desirably iron oriron and molybdenum and preferably these active metal catalysts aredeposited on the fibril aggregates as mixed oxides with alumina ormagnesia as described above.

The improved methods of making fibril-forming catalysts and the improvedcatalysts themselves substantially increase the yield of fibrils perunit of fibril-forming catalytic metal. Carboxylate treatment during themaking of fibril-forming catalysts yields catalysts with higherproductivity. Coprecipitation of catalyst metals(s) and aluminum ormagnesium compounds provides catalyst with higher loadings of activemetal(s) and therefore higher productivities. Further, use of fibrilaggregates as catalyst supports lend themselves to large scale fibrilproducing processes. The improved catalysts of the invention can be usedto produce not only fibrils such as are described in Tennent, U.S. Pat.No. 4,663,230—although the fibrils of the invention have higher purityas made—but also fibrils having different macromorphologies, such as theso-called fishbone (“FB”) morphology as described in published EuropeanPatent Application No. 198,558 to J. W. Geus (published Oct. 22, 1986).

DETAILED DESCRIPTION OF THE INVENTION

The term “fibril-forming catalyst” is used to refer collectively tocatalysts for forming discrete carbon fibrils, carbon fibril aggregatesor both.

The term “carbon fibrils” when referring to products is used to refercollectively to both discrete carbon fibrils and carbon fibrilaggregates, unless the context indicates a different meaning.

Carboxylate Treatment

Catalysts for producing carbon fibrils are made by incorporating aneffective yield-enhancing amount of an anion capable of an anionexchange reaction with OH-groups, such as a carboxylate or a phenolateinto a catalyst. Accordingly, catalysts for the production of carbonfibrils can be obtained by precipitating an effective amount of acompound of a fibril-producing metal, e.g. iron or iron and molybdenum,from an aqueous solution onto slurried particles of a support in thepresence of an effective, yield-enhancing amount of a carboxylate.Preferably the carboxylate may generally be an anion of a water-solublecarboxylic acid, such as a substituted or unsubstituted, mono-, di-,tri- or polycarboxylic acid, preferably a water-soluble monocarboxylicacid having 1 to 4 carbon atoms, or an aqueous emulsion of anonwater-soluble carboxylic acid with an emulsifier.

In alternative embodiments the carboxylate may be present inundissociated form provided that the carboxylate is capable of reactingwith or otherwise affecting the surface properties of the metal oxide ormetal hydroxide catalyst support. Hence, the carboxylate can be used ina nonaqueous solvent system that is also amenable to use as the solventsystem in the formation of fibril-forming catalysts, such as, forexample, an alcohol solvent system.

The carboxylate may be in the form of a hydrogen (i.e., carboxylicacid), sodium, potassium, ammonium or substituted ammonium, such asN-alkyl ammonium, carboxylate.

Desirably the anion is acetate or formate and is obtained from a watersoluble salt of formic acid or acetic acid. Other carboxylates includepropionates, butyrates, oxalates, citrates and tartrates.

A preferred form of the carboxylate is as the ammonium salt becauseammonium ion is destroyed upon drying. Other carboxylate salts such assodium, potassium and substituted quaternary ammonium, such as N-alkylammonium, may also be used.

A preferred method of carboxylate treatment includes the steps of (a)forming an aqueous solution of iron or iron and molybdenum salts, (b)forming a slurry of a catalyst support, e.g. alumina and or magnesia,(c) precipitating an iron compound or iron and molybdenum compounds ontothe slurried particles of alumina and/or magnesia in the presence of aneffective yield-enhancing amount of the carboxylate, preferably theanion of a lower carboxylic acid, at a pH at which precipitation isinitiated and the precipitate maintained in insoluble form and generallyin the range of 3 to 14, preferably 5.5-6.5, and (d) further processingit to produce the fibril-forming catalyst. Such further processing maycomprise separating the so-impregnated support material from the slurry,drying it and finely dividing it or injecting the slurry directly intothe reaction vessel for in situ conversion into catalyst.

In a preferred alternative embodiment, the method includes the steps of(a) forming an aqueous solution of iron or iron and molybdenum salt, (b)forming a solution of catalyst support precursor, such as an aluminum ormagnesium salt, (c) mixing these two solutions and coprecipitating amixed oxide catalyst in the presence of carboxylate and (d) separatingthe coprecipitated mixed oxide catalyst and further processing it. In avariant of this method, the alumina or magnesia support itself is formedin the presence of carboxylate and the catalyst is then precipitatedonto the thus-created support from an aqueous solution of the catalystprecursor(s).

Desirably the solution from which the compounds of fibril-forming metalare precipitated onto the slurried particles of support particlescontains from 0.04 to 4 grams of the anion of the carboxylic acid pergram of supported fibril-forming catalyst and preferably from 0.8 to 2grams. Where iron or iron and molybdenum is the active fibril-formingmetal, the weight ratio of anion of carboxylic acid to iron or iron andmolybdenum in the solution from which the iron or iron and molybdenum isprecipitated is broadly in the range of 0.07 to 14 and preferably is inthe range of 1.4 to 5.2.

In preferred embodiments where the carboxylic acid is acetic acid, theweight ratio of acetate to iron in the solution is in the range of 0.1to 5. Acetate is the preferred anion when precipitation is carried outat a controlled pH among other reasons because solutions thereof act asbuffers.

The precipitated metal on catalyst support is filtered and may be washedand reslurried and then vacuum or pressure filtered. The washed andfiltered slurry is then dried and may be ground to −100 mesh andthereafter tested for productivity. The catalyst is activated byreduction of the compounds of fibril-forming catalyst metal to thecorresponding catalyst metal. The catalyst can also be prereduced priorto use.

It has been found that it is advantageous to introduce the carboxylatesalt, e.g. of acetic or formic acid, into the slurry of the supportmaterial prior to its combination with the solution containing theactive fibril-forming metal compound.

While the precipitation procedure works best if the carboxylate ispresent in the slurry of catalyst support material, e.g. alumina, thesupport material can also be pretreated with a solution of carboxylateand then dried before the iron or iron and molybdenum salt isprecipitated thereupon. A post-precipitation contact with carboxylate isless effective, but still provides improved results over catalystswithout carboxylate treatment.

On magnesia-supported catalysts, still other procedures for treatingwith carboxylate work well. For example, washing a magnesia-supportedcatalyst with 1N ammonium acetate after precipitation of the iron oriron and molybdenum compound(s), i.e. a post-precipitation method, has astrong positive effect on catalyst productivity.

While not wishing to be bound by any theory, it is believed that thecarboxylate acts by exchanging with surface hydroxyl ions to alter thesurface characteristics of the support material. This, in turn, canstrongly affect the attachment of the small iron oxide oriron/molybdenum oxide particles to the surface of the support. Withmagnesia supports, the ion exchange can be accomplished efficiently bywashing after the iron oxide or iron and molybdenum oxides have beendeposited. It is believed that surface modification of the support bycarboxylate can be beneficial to the catalyst in two ways. First, thecarboxylate, by affecting the attachment of the metal oxide particles tothe surface, makes the subsequent reduction and activation of thoseparticles take place more efficiently. Secondly, the surfacemodification changes the friability characteristics of the supportsurface which can have a beneficial effect.

The improvement in yield achieved with carboxylate-treated catalystsranges from about 10 to 20% when preparing bird nest (BN) type fibrilaggregates to about 100% when producing combed yarn (CY) or open net(ON) type fibril aggregates.

Co-Precipitation

Improved fibril-making catalysts can be obtained by coprecipitating themixed oxide(s) of fibril-forming active metal and a precursor of asecond oxide, e.g. aluminum oxide or magnesium oxide. In a preferredembodiment an aqueous solution is formed comprising (i) a salt(s) of oneor more metal(s) having fibril-forming catalytic properties and (ii) analuminum and/or magnesium salt(s). The fibril-forming metal(s) and thealuminum and/or magnesium are coprecipitated from the aqueous solutionas the mixed oxides of the metal(s) and aluminum and/or magnesium andthereafter the precipitate is filtered, washed, dried and ground as isknown in the art.

Desirably the fibril-forming metal catalysts are iron or iron andmolybdenum and these are precipitated by the addition of a base such asammonium carbonate or sodium carbonate at a pH sufficient to initiateprecipitation and to maintain the precipitate in insoluble form. The pHis generally in the range of from 3 to 14 and preferably is in the rangeof 5.5 to 6.5. Precipitation preferably is carried out in the presenceof a yield-enhancing amount of an anion of a water-soluble carboxylicacid.

The solution from which the iron compound or iron and molybdenumcompounds are precipitated desirably contains from 0.01 to 1 gram ofiron, preferably from 0.3 to 0.5 gram; from 0.005 to 0.25 gram ofmolybdenum, preferably from 0.06 to 0.1 gram; and from 0.1 to 1 gram ofaluminum and/or magnesium, preferably from 0.2 to 0.5 gram, per gram offinished fibril-forming catalyst. The pH is sufficient to maintain theiron compound or iron and molybdenum compounds in solution is broadlyfrom 0 to 3 and preferably from 0 to 1.

Carbon Supports

Carbon particles having a high degree of structure and an open porestructure with high surface area can be used as supports for the mixedoxide catalysts of the invention. Carbon particles having a highporosity, i.e., a low bulk density, and a high surface area are said tohave a high degree of structure.

Carbon particles that are essentially pure carbon are preferred sincethey add the least amount of contamination to the final fibril product.Examples of these materials are (1) carbon blacks available from CabotCorp., such as the REGAL, VULCAN, MONARCH and ELFTEX series of carbonblacks described in Cabot Technical Report S-134 and (2) vapor-growncarbon fibers, such as reported by M. Endo, et al. in “The ExtendedAbstracts of the 18th Biennial Conference on Carbon”, Worcester, Mass.(American Carbon Society, University Park) at p. 151; by G. G. Tibbits,ibid. at p. 157, by Asahi Chemical Ind KK in Japanese Patent No.62-263,377 (May 6, 1986), by Showa Denka KK in Japanese Patent No.62-078,217 (Sep. 26, 1985); or by Nikkiso KK in Japanese Patent No.61-070014 (Apr. 10, 1986). These carbon fiber materials may also includepyrolytically deposited carbon and, optionally, may be graphitized in afurther processing step at >2500° C. Commercially availablereinforcement fibers, e.g., those made from poly(acrylonitrile) fibers,are also candidates. Any such material should be selected so as not toadversely affect the properties of the fibrils made using the materialas a catalyst support.

More preferred are carbon particles as described above with high surfacearea, i.e, >250 m²/g, since these are more amenable to use as catalystsupports, i.e, they possess the surface area, porosity, pore structuresand handleability necessary to prepare catalysts for commercialoperation. Examples of these materials are the activated carbons andactivated charcoals, such as the WV-B, WV-W, WV-L, or WV-G seriesmaterials available from Westvaco or the MONARCH BLACK PEARLS or VULCANXC72 materials available from Cabot Corp.

Most preferred are carbon fibril aggregates since they combine the highpurity and high surface area of the materials mentioned above and,additionally, present a uniquely high macro-porosity (up to 8 cc/g) withan open pore structure, i.e, a pore structure with essentially nomicropores (diameters >2 nm). Micropores often render the internalsurface area of a carbon material inaccessible to reactants because ofdiffusion limitations and make the particle often subject to plugging.Fibril aggregates with their unique porosity and relative absence ofmicropores do not suffer this shortcoming. Excellent results areobtained with fibril aggregates as supports for the mixed oxides offibril-forming metal(s) and aluminum or magnesium.

The alumina or magnesia can be formed in situ using the fibril aggregateas the sole support, or, aluminum or magnesium oxides can be added tothe aggregates as a well-dispersed slurry before deposition of theactive metal catalysts. The coprecipitation procedure described abovefor iron oxide or iron and molybdenum oxide and aluminum oxide may becarried out in the presence of the carbon particles or fibrilaggregates. Where an aluminum and/or magnesium compound and thefibril-forming metal are coprecipitated onto an aggregate or carbonparticle support, the aqueous solution of fibril-forming metal and analuminum and/or magnesium compound contains from 0.01 to 1, preferably0.2 to 0.5, gram of iron, from 0.005 to 0.25, preferably 0.05 to 0.1,gram of molybdenum and from 0.01 to 1, preferably from 0.2 to 0.5, gramof aluminum and/or magnesium per gram of supported fibril-formingcatalyst and the slurry of fibril aggregates or carbon particles hasfrom 0.01 to 0.9, preferably from 0.4 to 0.7, gram of aggregate orcarbon particles per gram of supported fibril-forming catalyst.

The aggregates are, for example, the BN, CY or ON aggregates describedabove although other aggregates are also useful, including aggregates offishbone (“FB”) morphology characterized by a crystalline graphiticstructure and a morphology defined by a fishbone-like arrangement of thegraphite layers along the axis of the filaments. Fibril aggregates aredesirable owing to their high surface area e.g., from about 250 to about1000 m²/gram and preferably greater than about 250 m²/gram, and theirunique macroscopic porosity of up to about 8 cc/gram, or typically inexcess of 1 cc/gram and preferably in excess of 5 cc/gram.

It has been found that an effective catalyst cannot be prepared byseparately coprecipitating the mixed metal oxides and then physicallymixing them with fibril supports. The latter procedure results in amixture of materials having greatly different bulk densities such thatwhen the mixtures are filtered, the solids partially separate and anon-homogenous mixture is obtained.

Coprecipitation in the presence of fibril aggregates gives a uniformhomogenous distribution of active catalyst strongly attached eitherchemically or physically to the aggregate. It is thought that the smallparticles of the metal oxides and the alumina and/or magnesia deposit inthe crevices and pores of the intertwining carbon fibrils in the fibrilaggregate and become strongly fixed thereto. The alumina or magnesiaprovides for the stable deposition of the iron oxide or iron andmolybdenum oxides. Were it not present, the iron and molybdenumparticles formed by reduction of the oxides would be too mobile on thegraphite surface of the fibril aggregate at the temperatures requiredfor fibril growth and would fuse into large particles and deactivatebefore starting to grow carbon fibrils.

A significant advantage of coprecipitation in the presence of fibrilaggregates, or, to a lesser extent, incorporating a fibril-forming metalonto magnesia particles in fibril aggregates, is that the amount ofalumina or magnesia in the catalyst is significantly reduced. Thisdecrease results in a higher yield of carbon fibrils based on alumina ormagnesia content and decreases the amount of alumina or magnesiaimpurity included in the product. While the amount of alumina ormagnesia decreases, the yield per unit of fibril-forming metal remainsthe same. The amount of washing that is needed to remove alumina ormagnesia from the carbon fibril product is also reduced.

Where magnesia is used, catalysts can be made by finely dispersingmagnesium oxide with fibril aggregates or carbon particles in water toform a slurry and adding a solution of iron or iron and molybdenumsalts, such as ferric nitrate and ammonium molybdate. Where thefibril-forming metal is incorporated onto magnesia particles in fibrilaggregates or carbon particles, the aqueous solution of fibril-formingmetal contains from 0.01 to 1, preferably 0.2 to 0.5, gram of iron andfrom 0.005 to 0.25, preferably 0.05 to 0.1, gram of molybdenum per gramof fibril-forming catalyst and the slurry of magnesia particles andfibril aggregates or carbon particles contains from 0.01 to 1,preferably from 0.2 to 0.5, gram of magnesia and from 0.01 to 0.9,preferably 0.4 to 0.7 gram of fibril aggregates or carbon particles pergram of supported fibril-forming catalyst. The homogeneity of theresulting catalyst is evident upon filtration. A well-dispersed,homogeneous solid catalyst has a regular coloration, while poorlydispersed catalysts, which undergo partial separation of solidcomponents on filtering, have black and gray striations in the filtercake.

Fine aqueous dispersion of magnesia is possible because of a physicalattraction between the magnesium oxide/hydroxide and the fibril surface.However, dispersability also depends on the starting material from whichthe magnesia dispersion is prepared. If the density of the magnesiumoxide is too high or the dispersibility is too low, non-homogeneouscatalysts can result. Therefore, procedures to fully disperse orhomogenize the fibril aggregates and magnesium oxide must be undertakenbefore neutralization of the iron or iron and molybdenum oxides iscarried out. Such methods of dispersion or homogenization are known tothe art.

Where alumina is used, catalysts can be made by dispersing fibrilaggregates in water and coprecipitating the oxides of iron, aluminum andmolybdenum from a solution containing ferric nitrate, aluminum nitrateand ammonium molybdate while maintaining the pH at 6.0±0.5 by concurrentaddition of a solution of ammonium carbonate. The homogeneity of thecatalyst is excellent as judged by coloration.

The supported fibril-forming catalyst of the invention comprises fromabout 1 to about 70 weight percent, preferably from 0.5 to 50 weightpercent and more preferably from 12 to 40 weight percent, of iron oriron and molybdenum; from 1 to 95 weight percent, preferably from 10 to85 weight percent and more preferably from 20 to 80 weight percent, ofalumina and/or magnesia; and from 1 to 90 weight percent, preferablyfrom 20 to 70 weight percent and more preferably from 30 to 50 weightpercent, of carbon fibril aggregates or carbon particles based on thetotal supported fibril-forming catalyst weight, with the proviso thatthe total weight percent of iron or iron and molybdenum alumina ormagnesia and carbon fibril aggregates or carbon particles does notexceed 100 weight percent.

The carbon fibrils of the invention that are made using the morepreferred supported fibril-forming catalysts have extremely high purity,as made—to the extent that they are essentially pure carbon. In thesecarbon fibrils the impurity level from the fibril-forming catalyticmetal is not more than about 1.1 weight percent, that from the aluminaor magnesia support is not more than about 5 weight percent and thetotal impurity level is not more than about 6 weight percent. Thisprovides a higher purity product. The previous levels of impurities inmaking fibrils were about 1.5 weight percent from the fibril-formingcatalytic metal, about 8.5 weight percent from the alumina or magnesiasupport and about 10 weight percent for total impurities. The purity ofthe fibrils as made in the invention reduces the amount of washingneeded to prepare the fibrils for sale and their cost. The impuritiesfrom the fibril-forming catalytic metal occur mainly as particles ofmetal that have been encapsulated within the carbon fibril so that theyare not exposed.

The inventions are further described in connection with the examples.

EXAMPLES I-IV describe methods for making fibril aggregates having birdnest, combed yarn and open net morphologies by methods known prior tothe invention.

EXAMPLES V-VIII are examples of the invention using carboxylate washsteps and they are comparative with EXAMPLES I-IV, respectively.

EXAMPLE IX describes an experiment to determine the optimum amount ofcarboxylate.

EXAMPLE X describes a method of coprecipitating a catalyst and support.

EXAMPLE XI describes production of a fibril aggregate-supported,coprecipitated catalyst.

EXAMPLE I

This example describes the preparation of a catalyst for making birdnest (BN) fibril aggregates.

A slurry of 800 grams of a gamma alumina (available from Degussa asOxide C) and 10 liters of deionized water was made up in a multi-neck,22 liter indented flask with rapid stirring. The pH of the slurry wasadjusted to 6.0.

A solution A was made by mixing 52 grams of ammonium molybdate [(NH₄)₆Mo₇O₂₄.4H₂O], dissolved in 500 milliliters of deionized water and 1500grams of 41% ferric nitrate [Fe(NO₃)₃] solution (9.5% Fe). Solution Aand a 20% by weight ammonium carbonate solution (Solution B) were addedconcurrently with rapid mixing to maintain the pH at 6.0±0.5. The pH wascontrolled by the relative rates of addition of Solution A and SolutionB. A silicone defoamer ANTIFOAM 289 available from Sigma Chemical wasadded in 5-300 ppm to suppress foaming during precipitation. Theaddition took about one hour, after which the resulting slurry wasvacuum filtered using Number 50 Whatman filter paper. The filter cakewas washed thoroughly twice by reslurrying in portions in a Waringblender for two minutes at medium speed with a total volume of 8 litersof deionized water followed by vacuum filtering. The conductivity of thesecond wash was about 1 mMho. The filter cake was dried at 162° C. in aconvection oven overnight. Samples were ground to −100 mesh and testedfor productivity.

The productivities of the catalyst for producing carbon fibrils wasdetermined in a 1 inch quartz tube reactor using the followingprocedure: A 1 inch quartz tube was fitted with a ¼ inch thermocoupletube inserted through the bottom. At the tip of the thermocouple tube aplug of quartz wool that had been previously weighed was placed whichpermitted passage of gas, but not particles of catalyst or fibrilsgrowing on the catalyst. The top of the quartz tube was fitted with agas line which allowed for a downflow addition of one or more gases, anda modified ball valve which allowed addition of a given charge ofpowdered catalyst. One opening of the ball was closed off so that itbecame a cup or sealed cylinder. Catalyst could then be loaded into thecup and the valve assembly sealed. The contents of the cup could then beadded to the gas stream without air contamination by turning the valve.

A thermocouple was inserted upward into the thermocouple tube to monitorthe reactor temperature. The tube reactor was heated to 680° C. in anArgon stream to purge the reactor after which the gas stream wasswitched to a mixture of hydrogen and ethylene at a flow rate of 400 and200 cc/min under standard conditions. A weighed charge of catalyst(about 0.02-0.05 g) was dropped into the downflow gas onto the quartzplug. The reactor was maintained at temperature for the about 20minutes, after which the reactor was cooled in argon and emptied. Theweight of carbon fibrils produced was calculated from the totalrecovered weight and the known weights of the quartz wool plug and thecatalyst fed. The yield of carbon fibril, or productivity, wascalculated as the weight of carbon produced per weight of catalyst orper weight of iron in the catalyst.

The yield based on catalyst was 19.5 and the yield based on iron contentwas 140.

EXAMPLE II

This example describes the preparation of a catalyst for making combedyarn (CY) fibril aggregates.

An aqueous slurry of a lightly calcined, finely ground hydrous alumina[Al₂O₃.3H₂O] (available from ALCOA as H705) was made in a 22 literreactor from 800 grams of the alumina support with 10 liters ofdeionized water, the pH was adjusted to 6.0 and after 0.5 hr vigorousstirring, the oxides of Fe/Mo were deposited as described in Example I.Solution A was made from 52 grams ammonium molybdate in 500 cc ofdeionized water and 1500 g of a 41% solution of ferric nitrate(available from Blue Grass Chemicals). Solution B was a 20% by weightsolution of ammonium carbonate.

Drying, washing and testing of the catalyst was carried out aspreviously described in Example I. The yield based on catalyst was 14.5and the yield based on iron content was 103.

EXAMPLE III

This example describes the preparation of a catalyst for making combedyarn (CY) fibril aggregates.

An aqueous slurry of a lightly calcined, finely ground activated alumina(available from ALCOA as CP2X), was made with 20.0 g of the support in300 cc of deionized water. The pH of the slurry was adjusted to 6.0 andthe slurry was vigorously stirred for 0.5 hr. Ammonium heptamolybdate,1.35 g, was dissolved in 25 cc of deionized water. Ferric nitratenonahydrate, 29.9 g, was dissolved in 100 cc of deionized water. The twosolutions were then mixed with stirring to form Solution A. As inprevious examples, the mixed Fe/Mo oxides were deposited into thesupport by concurrent addition of Solution A and a 20 wt % solution ofammonium carbonate at relative rates sufficient to maintain the pH ofthe slurry at 6.0±0.5.

Drying, washing and testing of the catalyst was carried out as describedin previous Examples. The yield based on catalyst was 13.2 and the yieldbased on iron content was 94.

Drying, washing and testing of the catalyst was carried out aspreviously described in Example I. The yield based on catalyst was 13.2and the yield based on iron content was 94.

EXAMPLE IV

This example describes the preparation of a catalyst for making open net(ON) fibril aggregates.

An 800 g batch of finely ground magnesia (available fromMartin-Marietta) was slurried with 12.5 liters of deionized water andheated to 75° C. for 2 hrs with vigorous stirring. The heat was turnedoff and the slurry allowed to cool.

58.4 grams of ammonium molybdate [(NH₄)₆ Mo₇O₂₄.4H₂O] were dissolved in300 ml of deionized water and the solution was mixed with 1824 grams of41% by weight ferric nitrate solution. This solution was added to theslurry with vigorous stirring. The pH of the slurry remained relativelyconstant at about 10.5-11.0 during the addition. The resulting slurryhaving red-brown color was filtered and washed twice with a total of 40liters of deionized water as in Example I and dried at 162° C.overnight. The dried catalyst was calcined at 400° C. in a convectionoven for 4 hrs, ground and sieved to −100 mesh and tested in the 1 inchquartz tube reactor using the procedure described in Example I. Theyield based on catalyst was 11.4 and the yield based on iron was 54.

EXAMPLE V

This example describes the improved performance of catalysts for makingbird nest (BN) fibril aggregates that have been carboxylate-treated.

An aqueous slurry of 800 grams of gamma alumina (available from Degussaas Oxide C) with 10 liters of deionized water was prepared as in ExampleI, after which 302 grams of 65% ammonium acetate aqueous solution (196grams active ammonium acetate available from HEICO Chemicals) was added.Additionally, 3 grams of a silicone defoamer ANTIFOAM 289 available fromSigma Chemical was added. The slurry was stirred vigorously for 30minutes, after which the method described in Example I was resumed todeposit the Fe/Mo oxides. Drying, washing and testing were done in thesame way. The acetate-to-iron mole ratio was 1. The yield based oncatalyst was 22.3 and the yield based on iron content was 160.

EXAMPLE VI

This example describes the improved catalyst performance of catalystsfor making combed yarn (CY) fibril aggregates that have beencarboxylate-treated.

A slurry was made with 800 grams of lightly calcined alumina support asdescribed in Example II, after which 450 grams of 65% by weight ammoniumacetate was added: and the procedure of Example II resumed. The yieldbased on catalyst was 34.2 and the yield based on iron content was 244.

EXAMPLE VII

This example describes the improved performance of catalysts for makingcombed yarn (CY) fibril aggregates that have been obtained withcatalysts that have been carboxylate-treated.

A 20.7 g sample of the lightly calcined activated alumina used inExample III was slurred in 300 cc of deionized water containing 5.1 gammonium acetate. The pH of the slurry was adjusted to 6.0 and theslurry was stirred vigorously for 0.5 hr. The procedure in Example IIIwas then followed using 39.2 g of 41% ferric nitrate solution and 1.3 gammonium heptamolybdate.

The yield based on catalyst was 18.2 and the yield based on iron was130.

EXAMPLE VIII

This example describes the improved performance of catalysts for makingopen net (ON) fibril aggregates that have been carboxylate-treated.

The procedure in Example IV was repeated, except that the wash liquidwas a 1 N solution of ammonium acetate. The rest of the procedureremained the same.

The yield based on catalyst was 21.9 and the yield based on iron was 76.

The comparisons between Examples I and V, II and VI, III and VII and IVand VIII, respectively, are set forth in Table 1.

TABLE 1 Acetate/Fe YIELD EXAMPLE (Wt Ratio) Morphology Catalyst Iron I 0BN 19.5 140 V 1.4 BN 22.3 160 II 0 CY 14.5 103 VI 2.1 CY 34.2 244 III 0CY 13.2 94 VII 1.4 CY 18.3 130 IV 0 ON 11.4 54 VIII N.A. ON 21.9 76

EXAMPLE IX

This example describes protocols for establishing the optimum amount andconditions of acetate treatment.

The optimum concentration for activation by ammonium acetate wasdetermined by varying the molar ratio of acetate ion to ferric ion inthe preparation procedure for making a combed yarn (CY) aggregate andthen measuring the productivities of the resulting catalysts.Productivities were determined by the procedure described Example I.

The productivities of the various catalysts for producing carbon fibrilswas determined in a one inch quartz tube reactor using the proceduredescribed in Example I.

The optimum concentration was determined for the CY catalyst made from acalcined, hydrous alumina support and is shown in Table 2.

TABLE 2 Acetate/Fe Fe YIELD YIELD RUN # (Wt Ratio) (%) (catalyst) (Iron)1 0.28 13.9 18.4 132 2 0.56 13.7 21.0 153 3 1.4 13.7 35.6 260 4 1.7513.6 36.6 269 5 2.18 13.4 34.5 257 6 2.8 13.4 37.2 277 7 5.2 13.4 26.9201 8 4.9 15.4 20.8 135

EXAMPLE X

This example describes the coprecipitation of the mixed metal oxidecatalyst with alumina in the preparation of fibril-forming catalysts.

Fresh solutions of 25 grams of ammonium molybdate [(NH₄)₆ Mo₇O₂₄.4H₂O],(available from GFS Chemicals) in 500 ml of deionized water and 489grams of ferric nitrate nonahydrate [Fe(NO₃)₃.9H₂O] (available from J TBaker in reagent grade) in 0.5 liters of deionized water were preparedand mixed with rapid stirring to give a clear, dark red-brown solution.This was then mixed with 816 grams of a 60% by weight solution ofaluminum nitrate nonahydrate [Al(NO₃)₃.9H₂O] (available from MineralResearch Development in technical grade). As needed, several drops of10% nitric acid were added until totally clear. This solution wasreferred to as Solution A.

A multineck 5 liter indented flask fitted with a mechanical stirrer anda pH meter was used for the co-precipitation of aluminum and iron oxidesat ambient temperature. Two liters of deionized water were added to theflask and the pH was adjusted to 6.0. Solution A and a 25% solution ofammonium carbonate (Solution B) were added concurrently with rapidmixing to maintain the pH at 6.0±0.5. The pH was controlled by therelative rates of addition of the two streams. The addition took about 1hour, after which the resulting slurry was vacuum filtered through No.50 Whatman filter paper. The filter cake was washed thoroughly twice byreslurrying in portions in a Waring blender for 2 minutes at mediumspeed with a total of 8 liters of deionized water followed by vacuumfiltering. The conductivity of the effluent after the second wash wasabout 1 mMho.

The filter cake was dried at 180° C. overnight in a convection oven. Theyield of dried catalyst was 194 grams with a calculated composition of49.8% Fe₂O₃, 10.5% MoO₃ and 39.7% Al₂O₃. The dried catalyst was ground,sieved to −100 mesh and tested in the 1 inch tubular reactor describedin Example I by the standard procedure described in that example. Twosamples gave yields after 20 min. of 43.5 and 42.8 based on catalyst.The iron content on the catalyst was 34.8% and the yield based on ironwas 124.

Table 3 shows the results of a series of catalysts prepared withdifferent Fe₂O₃/Al₂O₃ and Fe/Mo weight ratios.

TABLE 3 Co-Precipitated Catalysts COMPOSITION (%) Run Fe₂O₃/ Fe/Mo YIELD# Fe Fe₂O₃ Al₂O₃ MoO₃ Al₂O₃ (wgt.) cat Fe 1 14 20 76 4 .26 5 11.7 85 234 49 40 11 1.23 5 39 114 3 55 78 6 16 13 5 21 38 4 37 53 36 11 1.47 541 111 5 37 53 47 0 1.13 NA 24 65 6 39 56 32 12 1.75 5 45 114 7 41 59 356 1.69 10 45 110 8 37 53 44 3 1.20 20 46 124 9 35 50 40 10 1.25 5 43 123

The optimum Fe₂O₃/Al₂O₃ weight ratio appeared to be in the range from1-2 with a peak near 1.2. The optimum Fe/Mo weight ratio appeared to bea plateau ranging from 5-20.

Electron Microscopy (STEM) showed the fibrils produced to bepredominantly (>99%) carbon fibril. The morphology of the aggregate wasbird nest.

EXAMPLE XI

This example describes the preparation of a fibril aggregate-supportedcatalyst containing the mixed oxides of Fe,Mo and Al for making carbonfibril aggregates of a bird nest morphology.

A slurry of 28.5 grams of carbon fibrils was made with 500 millilitersof deionized water using a Waring blender for 2 minutes at high speed.The slurry was transferred to a multi-neck, indented 2 liter flask usinganother 500 milliliters of deionized water. The slurry concentrationafter transfer was about 2.8% by weight. The pH was adjusted to 6.0 andthe slurry was stirred vigorously with a mechanical stirrer for 0.5 hourat ambient temperature.

Ammonium molybdate [(NH₄)₆Mo₇O₂₄.4H₂O] (3.9 grams) was fully dissolvedin 50 milliliter of deionized water and mixed with 75.8 grams of a 40.0%by weight solution of ferric nitrate (available from Blue GrassChemical,) (Fe content=9.25%, by weight.) This solution was then addedto 122.5 grams of 60% (by weight) aluminum nitrate nonahydrate(available from Mineral Res. Devel.) to form Solution A. As needed, afew drops of 10% nitric acid were added to clarify the solutioncompletely.

Solution A and a 20% by weight solution of ammonium carbonate (SolutionB) in deionized water were added concurrently to the slurry withvigorous stirring. The pH of the slurry was maintained at 6.0±0.5 bycontrolling the addition of the two streams.

The solids were then vacuum filtered using No. 50 Whatman filter paperand the recovered cake was washed twice by reslurrying with 1 liter ofdeionized water in a Waring blender and refiltering. The filter cake wasdried at 180° C. overnight. Yield of dry catalyst was 49.2 grams with acalculated composition of 58.0% carbon, 20.4% Fe₂O₃, 6.5% MoO₃ and 15.1%Al₂O₃. A sample of the dry catalyst was ground and sieved to −100 meshand tested in the 1 inch tube reactor described in Example I. The fibrilyield was 21.0 based on catalyst and 148 based on Fe.

The results for this catalyst are given in Table 4, as follows:

TABLE 4 FIBRILS AS SUPPORT COMPOSITION (%) YIELD Fe Fe₂O₃ MoO₃ Al₂O₃ MgOC¹ cat Fe 14.2 20.4 6.5 15.1 0 58.0 21.0 148 ¹As bird nest carbon fibrilaggregate.

Electron microscopy indicated that fibrils grown in all cases werepredominantly bird nest (BN) fibril aggregates. Newly grown carbonfibril aggregate could not be distinguished from those that werecatalyst supports.

What is claimed is:
 1. A method for the production of carbon fibrilscomprising passing a fibril-forming feedstock under temperature andpressure conditions suitable for formation of fibrils over a supportedfibril-forming catalyst made by a process comprising the steps of:precipitating an effective amount of a catalyst compound of acarbon-fibril-forming catalyst metal from an aqueous solution onto aslurry of support particles in the presence of an effective amount of acarboxylate to enhance the amount of carbon fibrils produced per weightcatalyst used in a gas synthesis process compared with the amount offibrils which would have been produced in said gas synthesis process perweight of said catalyst made without said precipitating in the presenceof said carboxylate, wherein said metal-containing carbon-fibril-formingcatalyst compound comprises at least one metal derived from ametal-containing compound, said metal-containing compound beingdifferent from the carboxylate, and also being different from a compoundfrom which the carboxylate is derived, said carboxylate comprising ananion of a water soluble carboxylic acid.